As transistors are scaled to smaller dimensions there is a need for higher switching speeds. One solution to increase transistor speed is to strain the silicon in the channel. Adding a small amount of strain to the silicon lattice structure promotes higher electron and hole mobilities, which increase transistor drain current and device performance.
When the lattice is under tensile strain, its physical symmetry is broken, and with it the electronic symmetry. The lowest energy level of the conduction band is split, with two of the six original states dropping to a lower energy level and four rising to a higher energy level. This renders it more difficult for the electrons to be ‘scattered’ between the lowest energy states by a phonon, because there are only two states to occupy. Whenever electrons scatter, it randomizes their motion. Reducing scatter increases the average distance an electron can travel before it is knocked off course, increasing its average velocity in the conduction direction. Also, distorting the lattice through tensile strain can distort the electron-lattice interaction in a way that reduces the electron's effective mass, a measure of how much it will accelerate in a given field. As a result, electron transport properties like mobility and velocity are improved and channel drive current for a given device design is increased in a strained silicon channel, leading to improved transistor performance.
High tensile stress films have recently been introduced to the transistor device manufacturing process. For example, methods employing high-density plasma chemical vapor deposition (HDP CVD) to deposit a high tensile stress film using a two step process of (1) depositing using sources of silicon, oxygen and in many cases hydrogen; and (2) treating the film using a high density plasma or heat. However, these treatments are expensive because they reduce the throughput of the HDP tool used to deposit the film, and they may not penetrate deeply enough into the trenches to affect the film characteristics within the trenches.
Accordingly, new modification methods for increasing STI film tensile stress are needed.